The assembly and resin-encapsulation of a semiconductor power device commonly comprises the following steps:
fastening a die-stamped metal frame or strip of frames to a flat metal heat sink baseplate by wedging at least two tabs formed in two opposite sides of each die-stamped frame into indentations purposely formed in opposite sides of the baseplate so that once the wedging has been made, the metal frame or the strip of metal frames is supported by said wedged tabs parallel above the face of the baseplate and at a separation distance therefrom of some tenth of a millimeter; PA1 bonding the semiconductor die or dies onto the face of the heat sink baseplate in a central area in respect to the relative die-stamped metal frame so as to be surrounded by the inner ends of a plurality of coplanar fingers patterned in the die-stamped metal frame, which will constitute external connection pins, functionally isolated among each other and from the metal heat sink in the finished device; PA1 electrically connecting the metal pads of the front of the semiconductor die to the fingers of the metal frame, respectively, by means of a metal wire welded to a pad and to the inner end of a finger of the metal frame; PA1 transferring the assembly inside a mold and injecting an encapsulating resin which, upon solidification, "freezes" the various parts of the assembly in their respective spatial positions, thus isolating the fingers and the relative wire connections among each other as well as from the heat sink baseplate on which the encapsulating resin body is molded; PA1 separating the various devices, which may have been simultaneously encapsulated in a single mold, from the outer perimetral band of the die-stamped metal frame or frames, and separating the various external pins (i.e., the outer portions of the fingers of the die-stamped metal frame extending out of the cavity of the encapsulation mold, passing between the opposed mating surfaces of the mold and of the counter-mold).
Customarily, in accordance with modern fabrication techniques, the welding of the connecting wires to the metallized pads on the front of the semiconductor dies and to the respective finger or pin of the metal frame is performed by a thermo-sonic mechanism. Practically, the wire is pinned by means of a heated metal electrode against the metallic surface of the pad and of the finger's end. Mechanical vibrations of sonic and ultrasonic frequency are transmitted to the pinned wire through the heated metal electrode, causing a frictional, localized overheating which is sufficient to momentarily melt the metal so that, upon re-solidification, a permanent welding between the wire and the metallized surface of the pad and the surface of the metal finger, respectively, is established.
The flexibility of the fingers' inner end portion, which extends in a cantilever manner from an outer perimetral band of the die-stamped frame, under the force, though relatively small, which is exerted by the welding electrode, often represents a problem for reliably welding the wires. In some cases, this problem is overcome by inserting spacing laminae of a dielectric material in the gap between the surface of the flat heat sink baseplate and the fingers of the die-stamped metal frame, in order to support the cantilever ends of the frame fingers during the welding of the wires. These dielectric supports may also be left in place and be encapsulated in resin body. The insertion of these dielectric spacers or supports remains a time-consuming and expensive practice in terms of production costs and is impractical altogether.